1. Field of the Invention
The present invention relates to the use of stepping motors, particularly in the field of printing technology and of copier technology. In high-performance printers, for example, thus, a plurality of two-phase stepping motors are utilized for transporting a carrier material, which is preferably paper, from a paper input to a printing unit, past a fixing station up to a paper output. Continuous form paper or single sheet paper is utilized as the paper for printing.
2. Description of the Related Art
In addition to the advantages such as high step resolution and high positioning precision, however, stepping motors also have disadvantages. Thus, stepping motor controls are usually offered for three operating conditions. During standstill, the windings or, respectively, phases of the stepping motor are supplied with a low maintaining current. Upon acceleration of the stepping motor in an acceleration phase, a current having a very high current amplitude is utilized. When the stepping motor has reached its operating speed, then a switch can be made into a running operating condition wherein the current amplitude of the winding currents lies between that in standstill and that in the acceleration phase. An adaptation of the operating behavior to a time-variable load moment can only be roughly achieved, i.e. over the three operating conditions. In order to prevent the stepping motor from falling out of step, in minimum value of the torque reserve must not be downwardly transgressed either in the acceleration phase or in the running condition, this assuring that the difference of the drive moment and the load moment does not fall below a predetermined value. The greatest load moment occurring must thereby be taken into consideration; this, however, only occurs at times during operation of the stepping motor, so that the momentary torque reserve is usually higher than the predetermined minimum value. Accordingly, the stepping motor is supplied with too much energy. This energy is converted into disturbing fluctuations of the speed, into heat, running noises and other undesired side effects.
A known possibility for preventing these disadvantages is comprised therein that the amplitudes of the phase currents are varied such with the assistance of a control during operation of the stepping motor that the torque reserve respectively has exactly the predetermined minimum value. To that end, the load moment during operation of the stepping motor is determined, i.e. in real time, in that, for example, the actual position of the rotor of the stepping motor is constantly acquired with the assistance of an optical sensor unit. The amplitude of the phase currents and/or the load angle is employed as a manipulated variable. The outlay for constructing a complete control circuit with sensors, comparator means and actuator unit is disadvantageous. The demands made of the sensors and of the controller are generally extremely high since stepping motors should have a high positioning precision given a high running performance. For example, a relatively complicated four-quadrant control is employed as the current regulation in order to produce a symmetrical behavior both for building up the current a well as for dismantling, or decreasing the current.
A first aspect of the invention is directed to a method for the operation of a stepping motor wherein the load moment changes over time given an essentially identical speed. Such a situation occurs, for example in the high-performance printer, when a paper edge is drawn through between two fixing drums pressed against one another with extremely high pressure at the beginning of the fixing process. A sudden rise and subsequent, sudden drop of the load moment thereby occurs. In known high-performance printers, a brief-duration reduction of the speed of the fixing drums occurs due to the discontinuous course of the load moment, this leading to a discontinuous paper running. Given discontinuous paper running, the paper is more highly stressed compared to a uniform paper running. In the extreme case, tears in the paper to be printed can even occur. Given non-uniform paper running, moreover, dislocations of the paper relative to rated positions occur. Such dislocations can already lead to a visibly poorer printing quality in the range of a few hundredths of a millimeter.
U.S. Pat. No. 3,855,515 discloses that the time curve of the load moment be identified in a measuring procedure and that the amplitude values for the winding currents of a stepping motor be determined dependent on the curve of the load moment. Beginning from a starting time, the motor is supplied such with winding currents during operation that the motor consumes no more power then needed for driving the load. In this way, the dissipated power of the motor and of the control are reduced.
German Patent document DE 42 20 201 A1 discloses a conveyor means having at least one fixing drum that is driven by a stepping motor. A detector means acquires the transport speed of the transported sheet of paper, whereby this transport speed is kept constant with the assistance of the detector signals.